Requirements let you define how nodes relate to one another. For example, a
web_server node can be
contained_in a VM node or an application node can be
connected_to a database node.
<requirement_definition_name>: capability: <capability_type_name> node: <node_type_name> relationship: <relationship_type_name> occurrences: [ <min_occurrences>, <max_occurrences> ]
For more information, refer to TOSCA Requirement Section